Like many of you, when I was learning to sew, I was never told to cut off the lines that outline pattern pieces. I think most people I saw either cut outside them, or tried to go right down the center. When I finally heard that you should cut them off, I was surprised, but it sounded okay. I did wonder why the pattern companies never put that in the instructions. (Just one more surprise for us?) Perhaps they do now, I haven't looked.
So, I knew this was the thing to do, but it never really clicked in my head until I was doing these muslins for the coat and jacket I'm working on. I've been tracing around the pattern pieces with a Sharpie to mark the muslin, and it suddenly struck home that the reason you cut off the lines is because they're from tracing around the pattern pieces. It seems very obvious now, but I've never traced around pattern pieces to mark seamlines before except thread tracing on fashion fabric.
Since I'm doing Marfy patterns, there are no seam allowances on them so I'm not actually cutting off the lines, but I am stitching on the inside of the lines, which are fairly thick. I found the easiest way to get an accurate tracing from a paper patter (rather than an oaktag or other thicker pattern) is to use a slightly broader felt tip and run half of it on the pattern paper and the other half will fall on the muslin. That way there's no chance of sliding underneath the paper with the pen. When you're doing this, you're cutting single layer, although you can do double if you mark quite a few points with pins and turn your piece over after you've cut and then re-align your pattern and mark the second side. I do like to cut my fashion fabric single layer when it's important, though. It's easier to thread trace, and you don't have to cut all those tailor tacks apart.
What brought this on? I was looking at Fashion-Incubator and one of the archived posts featured today is about Marking & Cutting accurately. It's a classic, so read and enjoy if you haven't seen it, or even if you have, it's a nice refresher.